Nonlinear optical devices are used in a wide variety of commercial applications. For example, in the area of laser frequency conversion, a polymeric channel waveguide may be used for second-harmonic generation (SHG), that is, frequency doubling. Nonlinear optical devices may also be used for parametric amplification in optical repeaters, such as for optical signals at telecommunication wavelengths (1300 and 1550 nm), along a fiber optic communications route. Other examples of nonlinear optics include cascaded second order nonlinear devices for all-optical switching, bistability, spatial solitons, etc. Unfortunately, these applications typically require phase matching between the interacting fundamental and second harmonic waves.
A relatively successful conventional technique for phase matching is called quasi-phase matching (QPM). It involves a periodic modulation of the refractive index or the macroscopic nonlinear susceptibility .chi..sup.(2) such that the harmonic fields generated in different parts along the waveguide interfere constructively at the output. Quasi-phase matching has been demonstrated, for example, in inorganic ferroelectric crystal waveguides, such as disclosed, for example, by D. Eger et al., in Applied Physics Letters, 1994, volume 64, page 3208. The development of such crystals has been restricted because of the difficulty of crystal growth and its relatively high costs. See also U.S. Pat. No. 5,036,220 to Byer et al. which is directed to a non-linear optical waveguide in a solid state crystal body, such as LiNbO.sub.3.
U.S. Pat. No. 5,310,511 to Marcus discloses a planar polarizable body that is poled by placement between removable patterned plates which have conjugate patterns of openings therein and subjecting the openings to an electric field of the same polarity on opposite sides of the body. The electric field is sufficient to induce polarization of the body material in opposite directions through the thickness dimension with regions of alternate polarity at different lateral locations across the surface of the body.
In addition to ferroelectric crystals, poled polymer structures have been disclosed that contain chromophores with an extended .pi.-electron-conjugated system exhibiting large second-order nonlinearity. In the poled polymer structures the chromophores are aligned based upon their dipole moments by an electrostatic field, while the polymer is heated to near its glass transition temperature. The structure is then cooled and the molecules are frozen in the poled or oriented position. Poled polymers also offer the possibility of relatively straightforward processing and perhaps relatively low cost fabrication.
In particular, poled polymer waveguides have been described for use as electro-optic devices on an integrated circuit substrate where phase matching is not required. U.S. Pat. No. 5,058,970 to Schildkraut et al. discloses another quasi-phase matching optical waveguide including an array of laterally spaced transparent electrodes in direct contact with a transmission medium containing similarly polar aligned organic molecular dipoles in overlying areas. The transparent electrodes and overlying areas of the transmission medium are each of the same width and spacing. However, the guided beam suffers from significant losses as it passes through the region of the substrate covered by the ITO electrode grating.
In addition, U.S. Pat. Nos. 4,865,406 and 4,971,416 both to Khanarian et al. disclose a frequency doubling optical waveguide provided by a polymer thin film which has a periodic structure for quasi-phase matching of propagating laser wave energy. U.S. Pat. No. 5,061,028 also to Khanarian et al. discloses a poled polymer waveguide. The optical waveguide includes three electrodes, one of which has a spatial periodic structure, and a side chain polymer waveguide positioned between two side chain polymer cladding layers. The electrodes apply electric fields of opposite polarity to pole the polymer side chains bidirectionally in a noncentrosymmetric orientation of periodic reverse polarity.
Unfortunately, attempts to make polymer thin film waveguides having nonlinear properties, such as for frequency conversion, have been relatively difficult. In particular, it is has been especially difficult to accurately and precisely produce areas of alternating poling over many coherent lengths, and wherein each alternating area has a relatively large percentage of chromophores properly aligned.